Process for grinding particulate solids

ABSTRACT

A process is described for grinding or pulverizing particulate solids. The process comprises continuously introducing a fluid dispersion of solid particles as feed into the lower portion of a vertical cylindrical vessel containing a particulate grinding medium. The grinding medium particles are initially larger than and compatible in composition with the feed particles. The dispersion is agitated with the grinding medium to effect pulverization of the solid particles and the moving agitated dispersion and grinding medium is forced upwardly against a fixed suppression plate having a central opening. At this position the mixture is centrifugally classified whereby smaller particles pass through the suppression plate opening as product and the larger particles are retained in the vessel. In this way the grinding medium particles are used until they are totally consumed with particles thereof small enough to pass through the classifier leaving the vessel as part of the product. The grinding medium particles being lost from the vessel in this manner are replaced by fresh grinding medium.

United States Patent Nye [ PROCESS FOR GRINDING PARTICULATE SOLIDS [72] Inventor: John David Nye, Prescott, Ontario,

Canada [73] Assignee: Ferrox Iron Ltd., Prescott, Ontario,

Canada [22] Filed: Dec. 8, 1970 [211 Appl. No.: 96,127

[52] US. Cl ..24l/24 [511' Int. Cl ..B02c 17/16 [58] Field of Search....24l/24, 30, 78, 170, 172, 173, 241/184, 74, 46.17, 46.15, 68, 73,137,153,

' Primary Examiner-Robert L. Spruill AtzorneyPeter Kirby, Charles P. Curphey and Norris M. Eades 51 Oct. 17, 1972 [57] ABSTRACT A process is described for grinding or pulverizing particulate solids. The process comprises continuously introducing a fluid dispersion of solid particles as feed into the lower portion of a vertical cylindrical vessel containing a particulate grinding medium. The grinding medium particles are initially larger than and compatible in composition with the feed particles. The dispersion is agitated with the grinding medium to effect pulverization of the solid particles and the moving agitated dispersion and grinding medium is forced upwardly against a fixed suppression plate having a central opening. At this position the mixture is centrifugally classified whereby smaller particles pass through the suppression plate opening as product and the larger particles are retained in the vessel. In this way the grinding medium particles are used until they are totally consumed with particles thereof small enough to pass through the classifier leaving the vessel as part of the product. The grinding medium particles being lost from the vessel in this manner are replaced by fresh grinding medium.

6 Claims, 1 Drawing Figure PROCESS FOR GRINDING PARTICULATE SOLIDS BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a new and improved milling process for pulverizing and dispersing solid particles.

2. Description of Prior Art Methods for dispersing solid materials and reducing the particle size by agitating a dispersion of the solid material with a grinding medium are well known and widely used. One example of such a process is described in Talpey Canadian patent No. 771,141, issued Nov. 7, 1967. Various devices are also known for this purpose such as the agitator ball mill available from NETZSCH under the trademark Molinex" as 'well as a continuous attritor available from Union Process, Inc. of Akron, Ohio. These devices usually take the form of a vertical circular container having therein a rotatable member for agitating the dispersion of grinding medium and solid material. In operation, a dispersion of the solid material is introduced continuously into the bottom of the apparatus while the rotatable member agitates the mixture with the grinding medium. The material overflowing the top of the apparatus passes through one or more screens which collect the grinding medium but allow the solid material now reduced in size to pass through. The grinding medium collected at the screen is periodically returned to the system.

The grinding medium may be sand as described in Canadian patent No. 771,141 or it may be in the form of balls of various material, e.g., iron, aluminum oxide, glass, porcelain, etc.

The use of screens for separating grinding medium from the pulverized material has always presented problems. Thus, the pulverized material is very fine, e.g., l-50 microns, and it becomes virtually impossible to separate this from fine particles of grinding medium, particularly when the pulverized material is in the form of a very thick slurry. It will, 'of course, be appreciated that a thick slurry is highly desirable since this decreases the total amount of material passing through the system and also is an important consideration in efficient grinding or pulverizing.

The above mentioned Canadian patent describes a type of internal classifier which separates the grinding medium from the pulverized material by means of-centrifugal action and a suppression plate. However, it is concerned with avoiding any-carryover of sand intothe product obtained and because of this it was found necessary to use a screento collect the fine particles of sand. Moreover, in order for the sand to be utilized for any extended period of time it was important that the materials being pulverized are softer than the sand. Otherwise,the sand would break down into smallparticles which would be carried out of the system and large amounts 'of this would quickly clog the screen.

It is, therefore, the purpose of the present invention to provide an improved process for pulverizing solid materials which particularly will avoid the need for a screen.

SUMMARY OF THE INVENTION According to thepresent invention a processis provided for grinding or pulverizing particulate solids which comprises continuously introducing a fluid dispersion of solid particles as feed into the lower portion of a vertical cylindrical vessel containing a particulate grinding medium. The grinding medium particles are initially larger than and compatible in composition with the feed particles. The dispersion is agitated with the grinding medium to effect pulverization of the solid particles and the moving agitated dispersion and grinding medium is forced upwardly against a fixed suppression plate having a central opening. At this position the mixture is centrifugally classified whereby smaller particles pass through the suppression plate opening as product and the larger particles are retained in the vessel.

With the process of this invention, by utilizing grinding medium particles which are compatible in composition with the feed particles,-it is possible to continue to use the grinding medium particles until they are totally consumed. Thus, particles of grinding medium small enough to pass through the centrifugal classifier leave the vessel with the product. These small particles are formed by breaking down and wearing down the initial larger grinding medium-particles. As the grinding medium is gradually used up in this manner, fresh grinding medium particles are added to the system.

The process of this invention has the great advantage that it permits the system to be kept in operation continuously over long periods of time and it is only necessary to keep adding fresh grinding medium to replenish that which has been used up and carried out with the product.

be used for pulverizing ferric oxideor ferrites, porcelain balls can be used for pulverizing porcelain particles, etc. The grinding medium particles are typically balls having an initial diameter of one-sixteenth to onehalf inch, while the material being pulverized will normally have an initial particle size'of less than 14 mesh (U.S. Sieve).

The grinding of hard magnetic materials such as bariumand strontium ferrites represents a particularly useful function of the present process. These hard ferrites are widely used because of their excellent magnetic properties and their case and cheapness of production. The ferrites are used in anisotropic magnets for speaker systems, as adhesion magnets and ring or segment magnetsinmotors and generators, for magnetic couplings, etc.

The magnetization and coercive force of finished anisotropic ferrite magnets are determined by the calcining and the final sintering temperature, the milling process to which the powder is subjected and their chemical composition. In particular, the pulverizing stage is of the greatest importance in achieving good magnetic properties and this is one of the difficulties in the production of these products. Thus, in producing high grade hard ferrites, barium, strontium or lead carbonate-and iron oxide are mixed together and are calcined at high temperatures. During this stage crystals of hard ferrite are formed. The presintered product is then coarsely crushed in crushers and this is followed by a pulverization process, usually by wet milling. The

ferrite material thus obtained inthe form of a slurry is then either dried and pressed dry as a powder or granulate, to its final magnet form, or, after a part of the liquid has been removed from the slurry it is pressed wet or extruded. For the production of anisotropic magnets, pressing is carried out in a magnetic field. Subsequently, to ensure mechanical strength and the desired magnetic properties, the magnets are sintered at about l,200C.

Pressing in a'magnetic field causes all the powder particles to take up a preferred direction, thus conferring a high degree of magnetization. The more complete the orientation of the particles, the better will be the magnetic properties. This calls for powder possessing certain specific properties. In particular, it is important for the powder particles to be very small, usually having a size of less than one micron..On the other hand,it is important that the particles are not too small, which means that the milling process must be capable of yielding a very fine powder within a narrow particle size range. I

The process of this invention is particularly well adapted to this pulverizing operation. Thus, we are here concerned with a large scale commercial operation in which large amounts of a hard ferrite are being continuously processed and it is essential for an economic operation that the pulverizing process can be continued for long periods of time without interruptions. in order to properly pulverize the hard ferrite, a very thick slurry, e.g., 50 percent solids, is fed to the pulverizer and it is extremely difficult to separate this thick slurry from iron milling balls by means of a screen. However, since the hard ferrite product can tolerate small amounts of iron, with the process of this invention it is possible to do without screens altogether and simply rely upon an internal centrifugal classification whereby particles of, for example, less than 1 micron are allowed to pass out of the pulverizer while particles larger than this size are retained in the system. With this arrangement the iron balls can be continuously used and are retained within the system by the classifier and consumed until they have a particle size of less than 1 micron. Thus, particles of iron resulting from abrading of the iron balls leave the system together with the hard ferrite powder. As the total volume of iron balls in the system decreases, fresh balls are added.

in order to more clearly describe this invention, reference is made to the accompanying drawing which illustrates an embodiment of an apparatus suitable for carrying out the process. The drawing is a vertical section of a simple apparatus for continuous operation.

Referring to the drawing, the apparatus for pulverizing solid materials comprises a cylindrical vessel I having a bottom 2. For ease of dismantling, the vessel 1 is in three parts joined together by means of pipe flanges 3 and 4. In the upper region of the vessel 1 is an overflow weir 5 to which is connected an outlet conduit 6.

Below the weir 5 is an annular suppression plate 7" which is fixed on the inner wall of the vessel. This suppression plate has an axial opening 8. At a spaced location below suppression plate 7 is another suppression plate 9 mounted on the inner wall of vessel 1 and this again has an axial opening 10. This suppression, plate is just above the flange 3. t

Extending downwardly through the openings 8 and 10 passes a motor driven shaft 1 1 to which is connected an impeller shaft 11a. The impeller shaft has attached thereto a series of spaced paddles on impellers 12. Also mounted on shaft 1 1' directly below suppression plate 7 is a flat, horizontal circular disc 13 whileanother flat horizontal circular disc. 14 is mounted a on shaft 1 la directly below suppression plate 9 by means of a set screw collar 20.

Extending at an angle downwardly through the wall of vessel 1 is a stand pipe 15 for feeding grinding medium to the vessel. The lower end of this stand pipe terminates adjacent the top face of rotary plate 14 and this pipe has a small cut-out 16 at the trailing edge in the direction of rotation of disc 14. With this cut-out balls which are travelling down pipe 15 are carried away by the rotation of disc 14 through the opening 16 but are knocked off the disc 14 by the opposite side of pipe 15 as they rotate around.

Adjacent the bottom of vessel 1 is a slurry inlet line 17. The vessel 1 can be surroundedby a water jacket for cooling purposes if necessary. v I

In operation, the pulverizing of solid material is accomplished by pumping the slurry into the bottom of vessel 1 through inlet line 17. The proper amount of iron balls is added to the system at start-up and the shaft 11 set in rotation. As pulverization takes place by the action of the impellers on the iron balls and solid particles, the dispersion is forced upwardly by the pump and first passes through opening 10. The larger iron balls tend to remain towards the bottom of the vessel because of gravity, but particularly as these balls become smaller they tend to be carried up and the rotating disc 14 serves the dual function of centrifugally separating the large particles, including both iron balls and large particles of material being pulverized, from the finer particles which pass through the opening and up into the region 18. The large particles thrown off to the side by the disc 14, of course, fall down the walls of vessel 1 and back into the pulverizing zone.

The dispersion which has passed throughoutlet l0 continues to move upwardly where it comes in contact with the second rotating disc 13 and suppression plate 7 at which location a second classification takes place.

-At this region any larger particles which have managed to pass through opening 10 are thrown to the side by disc 13 and the dispersion containing only fine particles passes through outlet 8 and over weir 5 down outlet pipe 6.

The suppression plates and outer walls are. preferably made of steel although they may be surfaced with rubber or other materials. The impellers may be of any desired shape such as flat or curved circular discs, propellers, paddles, or other stirring means. The number of impellers and their relative spacing is not critical provided the stirring is capable of imparting motion to the balls and dispersion throughout the milling vessel. It is, however, preferable that the vessel be tall relatively to its diameter and that a number of impellers are provided along the height of the pulverizing zone. The reason for this is that the impellers are subjected to considerable wear by the action of the balls and material being pulverized and it has been found that the operation can be commenced with the balls in the lower part of vessel 1 so that the impellers at the lower end of shaft 11 are worn out first. As these lower impellers wear out, the high impellers can be made to take over by increasing the amount of balls in 11 by means of the tapered screw connection 19. This entire lower section with the plate 14 attached is then removed, the plate taken off, placed on a fresh impellor section and the entire mechanism reassembled.

The most critical dimensions of the system are the diameters of the openings 8 and 10 as well as the spacing between the rotating plates 13 and 14 and the corresponding suppression plates 7 and 9. These can be varied depending on the material being processed, the product desired, etc., and the precise conditions can be easily determined by simple experiments.

With a fixed set of dimensions as discussed above, the particle size of the pulverized product can be varied by varying the throughput of slurry of particulate solids with the mill working at constant speed. Also, with a fixed throughput of particulate solids slurry, the grinding medium requirements can be determined by the electric power load on the drivemotor. Thus, the grinding medium can be added during the operation in amounts sufficient to maintain a constant load on the drive motor.

In the embodiment shownin the drawings the vessel 1 is a 20 inch outside diameter standard steel pipe with half inch walls. The impellers-used are in the form of steel bars 16 inches long and 1% inches in diameter. Flange 4 is positioned 30 inches from the bottom 2 while flange 3 is positioned 30 inches above flange 4. The plates 13 and 14 are lfi-inch thick steel and are dimensioned such as to provide a 22-inch peripheral clearance between their edge and the inside wall-of vessel l. The suppression plates 1 and 9 both have a 2-inch diameter axial opening and each rotating plate ispositioned 1 inch below its corresponding suppression plate. The stand pipe is a 3-inch diameter steel pipe and this extends 60 inches above flange 3.

The device is driven by a 75 h.p. 440 volt motor at a speed of about 300-350 rpm. The starting material being fed to the device is usually less than 60 mesh while the iron balls being fed in are about one-eighth inch diameter, With this system about 500 to 1,000 pounds per hour of hard ferritescan be processed. At this rate of operation it is necessary to providecooling in order to maintain the temperature of this system below the boiling point.

The invention is further illustrated by the following non-limiting examples.

EXAMPLE 1 a. A slurry feed was prepared containing 50 percent by weight of barium ferrite particles in water. About 75 percent by weight of these particles were less than 325 mesh (U.S. Standard Sieve).

This slurry feed was pumped through the above described mill at a rate of 500 pounds of solids per hour with the impellers rotating at a speed of 350 rpm. The

grinding medium was carbon steel shot having an initial diameter of about 0.18 inch and sufficient shot was maintained in the mill to keep a load of -70 amps on the drive motor. This required an addition of about 20 pounds of shot per ton of barium ferrite throughput. The shot was added through the standpipe in 50-pound allotments about every 4 to 6 hours when the load on the motor fell below the minimum.

The barium :ferrite slurry emerging from the mill contained ferrite particles having an average particle size of about 1' :micron. This slurry was pumped to a stand tank where the barium ferrite was permitted to settle as a mud. The barium ferrite mud was used in the wet form to produce orientedmagnets or dried to a fine, dry powder to be used-for making unoriented magnets.

-b. To produce-the oriented magnets, the barium ferrite mud was pressed in a mould at a pressure of about 3 tons per square inch in a magnetic field which established the desired field orientation. After compaction .had been completed, the pressed bodies having oriented barium ferrite crystals were transferred to a sintering oven where they were sintered at, temperatures of about 220 to 2,450F.

lf unoriented magnets are required, then the above mentioned dry barium ferrite powder is pressed in a mould at a pressure of about 6 tons per square inch, after which the compacted bodies are sintered under conditions similar to those described above for oriented magnets.

In terms of coercive force (H) and energy product (Bl-i), the sintered magnets obtained from the barium ferrite produced according to this invention compare favorably with the best products at present available commercially.

EXAMPLE 2 All of .the grinding conditions of Example 1 were maintained,-with the exception that the slurryfeed was pumpedthrough the mill at double the rate, i.e., 1,000 poundsof solids per hour. The slurry product from the mill was collected ina hold tank and then fed through the mill a-second time at a-feed rate of 1,000 pounds of solids per hour.

This twostage grinding at the same average feed rate as Example 1 (500 pounds of solids per hour) produced a barium ferrite slurry having a greater uniformity of particle size than wasobtained from Example 1. The improved particle size uniformity resulted in even better magnetic characteristics in the sintered magnets thenwere obtained in Example 1.

l claim asmy invention:

1. A process for grinding particulate solids in a grinding mill having a lower vertical cylindrical grinding zone and an upper separating zone, said zones being separated by a fixed suppression plate having a central opening,

said process comprising the steps of continuously introducing a fluid dispersion of solid particles as feed into said grinding zone containing a particulate grinding medium, said grinding medium particles being initially larger than and compatible in composition with the feed particles, agitating said dispersion with the grinding medium, forcing the moving agitated dispersion and grinding medium mixture upwardly against said suppression plate,

centrifugally classifying the mixture below the suppression plate whereby smaller particles pass through the suppression plate opening into the separating zone and larger particles are retained in the grinding zone, further centrifugally classifying the particles in the the separating zone, continuously removing smaller particles collected from the separating zone classification as product and during continuous operation of the mill adding fresh grinding medium particles to replace those worn down and discharged with the product, said particles entering the vessel by being released from a stand pipe onto a substantially horizontal rotating disc in the grinding zone. I 2. A process according to claim 1 wherein the grinding medium particles are balls having an initial diameter of about one-sixteenth to one-half inch.

3. A process according to claim I wherein the feed particles have a size of less than 14 mesh.

4. A process accordingto claim 1 wherein the pulverized product has an average particle size of lessthan 1 micron.

S. A process according to claim 1 wherein the feed is particles of a hard ferrite and the grinding medium is iron balls.

6. A process according to claim 1 wherein the grinding is conducted in two stages, with the dispersion feed rate in each stage being double that required to produce a final pulverized product of a predetermined average particle size.

a c v a a e 

2. A process according to claim 1 wherein the grinding medium particles are balls having an initial diameter of about one-sixteenth to one-half inch.
 3. A process according to claim 1 wherein the feed particles have a size of less than 14 mesh.
 4. A process according to claim 1 wherein the pulverized product has an average particle size of less than 1 micron.
 5. A process according to claim 1 wherein the feed is particles of a hard ferrite and the grinding medium is iron balls.
 6. A process according to claim 1 wherein the grinding is conducted in two stages, with the dispersion feed rate in each stage being double that required to produce a final pulverized product of a predetermined average particle size. 